Locksmiths typically use several relevant terms interchangeably when referring to particular features associated with locks. To avoid confusion the following terms will be assumed to be used in the following sense throughout the description and claims.
In a lock of the pin tumbler or similar type, the lock comprises a central plug and an outer housing or shell, in which the plug rotates, the housing being mounted in a suitable latch structure. The terms plug and housing will be used in this context. Similarly, it will be appreciated that the present invention may be applied to any type of latch structure for which pin tumbler and related locks are used, for example, cam locks, door locks, phone locks and security fittings.
A number of prior disclosures describe devices intended to provide a removable plug or lock core arrangement. The advantages of removable plug locks is that they may be rapidly re-keyed by relatively unskilled personnel. Such re-keying may be required as a result of a potential security breach occuring due to the loss or non-return of a key. The present invention is particularly concerned with locks of the type in which the plug contains the complete combination of coded pins or similar elements. An example of this type of lock is the BILOCK.TM. device sold by the applicant. The keying of such locks can be altered by simply replacing the plug without involving any other elements. Locks of this type typically have one or more side bars extending between the plug and the housing in the locked condition to engage a complementary recess formed into the inside surface of the housing thereby preventing plug rotation and associated latch movement.
One type of lock featuring a removable keying mechanism is disclosed in U.S. Pat. No. 5,226,304 to Scott in which is described an arrangement for removing an entire lock core consisting of a plug surrounded by an interchangeable bracket, with radial projection housing the lock tumblers. In its retained state the core is held to the surrounding housing by two sets of balls forced by the cylinder body to protrude through crimp holes in the core bracket so as to engage ball grooves in the surrounding housing. Core removal is by withdrawal of the retaining balls, one set of balls being withdrawn into notches of a removal key and another set being withdrawn into dimples in the outer surface of the cylinder body on rotation of the cylinder. In contrast the present invention is simpler and relies on retaining balls operating directly between the plug and the housing. In a retained state according to the present invention one set of retaining balls located in the plug is forced into engagement with a groove in the housing by the insertion of an operation key thereby preventing plug withdrawal.
Another group of known removable core devices uses an extended removal key to facilitate core removal in contrast to a shorter standard length operation key which is used during normal operation of the lock. A removable core lock according to this scheme has a special pin distal from the keyway entrance and accessible by the removal key. Such an arrangement necessitates a longer housing assembly than is required in a non-removable core lock and hence in many cases cannot be retrofit on to existing systems. Examples of this type include U.S. Pat. No. 4,715,201 to Craig, U.S. Pat. No. 4,191,037 and U.S. Pat. No. 4,398,405 to Patriquin, U.S. Pat. No. 5,101,649 to Duval, and U.S. Pat. No. 5,121,618.
Another approach has been to use pins with multiple tumblers so that more than one shear line can be generated. In devices of this type insertion of an operation key generates a shear line allowing operation of the lock but not removal of the core. Insertion of a removal key produces a different shear line which transmits force to a withdrawal mechanism. An example of such a system is disclosed in U.S. Pat. No. 5,507,163 to Juang in which a key assembly is held to the door handle it is to be inserted in by flanges on a ring located midway down the length of the lock assembly. The ring may be rotated due to the action of a special key and multi-segmentation of the middle two lock tumblers. These tumblers form a shear line which selectively transmits rotational force applied to the key to the operative ring thereby rotating the ring to a position in which disengagement is possible. Similarly in U.S. Pat. No. 3,667,264 to Surko a control key generates two shear lines positioned to allow a medially placed engagement latch to be rotated into and out of the core. When rotated into the core the lock assembly can be withdrawn from its housing.
It is an object of the present invention to provide an arrangement for removing only the plug of a lock so as to facilitate re-keying, and further for the invention to be installable within a conventional latch mechanism. By the invention two types of key are provided a first operation key for standard locking and unlocking of the lock and a removal key by which an operator may withdraw the plug from the housing.